The influence of various welding wires on microstructure, and mechanical characteristics of AA7075 Al-alloy welded by TIG process

Owing to their exceptional mechanical properties, the various welding wires used to combine aluminum can meet the needs of many engineering applications that call for components with both good mechanical and lightweight capabilities. This study aims to produce high-quality welds made of AA7075 aluminum alloy using the GTAW technique and various welding wires, such as ER5356, ER4043, and ER4047. The microstructure, macrohardness, and other mechanical characteristics, such as tensile strength and impact toughness, were analyzed experimentally. To check the fracture surface of the AA7075 welded joints, the specimens were examined using optical and scanning electron microscopy (SEM). A close examination of the samples that were welded with ER5356 welding wire revealed a fine grain in the weld zone (WZ). In addition, the WZ of the ER4043 and ER4047 welded samples had a coarse grain structure. Because the hardness values of the welded samples were lower in the WZ than in the base metal (BM) and heat-affected zone (HAZ), the joints filled with ER5356 welding wire provided the highest hardness values compared to other filler metals. Additionally, the ER4047 filler metal yielded the lowest hardness in the weld zone. The welding wire of ER5356 produced the greatest results for ultimate tensile stress, yield stress, welding efficiency, and strain-hardening capacity (Hc), whereas the filler metal of ER4043 produced the highest percentage of elongation. In addition, the ER4047 fracture surface morphology revealed coarser and deeper dimples than the ER5356 fine dimples in the welded joints. Finally, the highest impact toughness was obtained at joints filled with the ER4047 filler metal, whereas the lowest impact toughness was obtained at the BM.


Materials and methodology of research Materials
This study concentrates on a commercial Al-Zn-Mg aluminum alloy, specifically with a thickness of 6 mm, provided by the Egyptian Aluminum Company, Egypt.It involves the use of distinct welding wires made from aluminum alloys of grades ER5356 (Al-Mg), ER4047 (Al-Si), and ER4043 (Al-Si) provided by the ESAB Company, Cairo, Egypt, each with a diameter of 3.2 mm.The composition and mechanical properties of the base material and filler metals (according to the datasheet provided by the ESAB Company) are detailed in Tables 1  and 2, respectively.The experimental plan for this study is illustrated in Fig. 1.

Welding process
Using Wire-cut Electrical Discharge Machining (WEDM), AA7075 BM was prepared for GTAW in a flat position with dimensions of 150 mm in length, 100 mm in width, and 6 mm in thickness.To create a single V-groove with a 60° angle, 2 mm root gap, and 2 mm root face, each sample was milled to a 30° angle using a milling machine, as shown in Fig. 2. Prior to welding, the BM was cleaned with acetone and a wire brush to remove any inclusions, grease, grime, and oxides.Butt welds were created using a fixed current and voltage in the GTAW process with DCEN polarity.Pure argon gas (99.99%) at a flow rate of 10 L/min was used to protect the welding pool.In this work, a 2.4 mm diameter non-consumable AWS EWTH-2 tungsten electrode was utilized.The fabricated similar butt welds were examined for the microstructures of different welding regions.Metallographic samples were prepared by cutting the transverse section across the weld line, as shown in Fig. 3a.The microstructure samples were ground using different degrees of emery papers (800, 1000, 1200, 1500, and 2400 grit), followed by polishing with a 0.3 μm Al 2 O 3 paste.Microscopic investigations of Al-Zn-Mg alloy butt welds were carried out by etching with a ified Keller's reagent (2 mL of HF, 3 mL of HCl, 5 mL of HNO 3, and 90 mL of H 2 O).The microstructure of the different zones of interest, such as the WZ, HAZ, and BM combination, was viewed and captured with an optical microscope (OM, Nikon Instruments Inc., Tokyo, Japan) coupled with the analysis of these images using software.The grain sizes for all welding joints produced were measured from photomicrographs obtained in the polarized light of a light microscope (LM) using the random secant method.

Mechanical characterization for welded joints
To analyze the mechanical behavior of all welding specimens, the tensile properties, including UTS, YS, and El%, as well as the impact test, were carried out at room temperature on a universal testing machine (WDW-300D, Guangdong, China).A tensile test conducted in accordance with ASTM E8M-04 was utilized to ascertain the mechanical behavior of the welded alloy AA7075; see Fig. 3c.On the polished cross-sections, the macrohardness profiles were also measured using a Vickers hardness tester machine (HWDV-75, TTS Unlimited, Osaka, Japan) with a 0.5 kg load and a 15 s dwell time at all various welding locations.As per ASTM E384-09, hardness test samples were utilized for every welding zone; Fig. 3b illustrates this.Finally, a JB-300B Charpy Pendulum Impact Testing Machine was used to measure the impact energy at room temperature.As shown in Fig. 3d, the impact test samples were cut and prepared in accordance with ASTM E23-01.

Visual inspection of weldments
Figure 4 shows the effect of various filler metals (ER5356, ER4043, and ER4047) on the quality of the weldments produced.In addition, all welding variables allowed us to produce weld joints with free defects, such as cracks and porosity.The welded joints produced by ER4043 had thin and smooth welding lines, whereas those produced by ER5356 and ER4047 welding wires had thick welding lines.This was the most noticeable difference in the  visual appearance of the welded joints.Based on a visual examination of the production welding specimens, no defects appeared in the weld bead shape on both the face and root sides.Thus, it is impossible to say with absolute certainty that the welding wires used with ER5356, ER4043, and ER4047 had any effect on the occurrence and formation of welding defects.Figure 4a-c displays the morphology of the welded joints produced with various types of welding wires.Additionally, all welded joints were visually inspected to ensure that the samples were defect-free.Table 3 indicates the visual inspection report results for the AA7075 aluminum alloy joints produced using various welding wires: ER5356, ER4043, and ER4047.Based on the visual inspection results, all the welded joint surfaces were considered acceptable.However, a few points should be noted for manufacturing welded joints such as face joint reinforcements, which vary in 1.28-1.56mm thickness, as presented in Table 3.
According to the positive visual examination results, the selected welding wires are perfect for joining AA 7075 aluminum alloys using TIG welding.

Microstructure of weldments
Figure 5a illustrates the microstructure of AA7075's initial material, which represents elongated grains along the rolling direction, accompanied by light-gray FeAl 3 particles and spheroidal MgZn 2 particles (black precipitates) in the aluminum solid solution 28 .Hence, the aluminum alloy of the 7XXX series was significantly influenced by its secondary phases.In addition, numerous studies have thoroughly investigated and detailed the nature of typical secondary phases [29][30][31] .The 7XXX family of aluminum alloys primarily contains four types of intermetallic compounds as significant secondary phases: η-phase (MgZn 2 ), S-phase (Al  and SiO 2 ), and Mn-rich phases (Al 6 (Fe, Mn), Al 5 Si 2 (Fe, Mn), and Al 3 (Fe Mn Cr)) would also form in the alloys owing to the presence of Mn, Fe, and Si 32 .Additionally, the density, size, shape, and distribution of the intermetallic phases that are frequent in AA7075 Al alloys have a significant effect on the mechanical behavior, as well as the alloys' tendency toward solidification cracking [32][33][34] .In addition, the initial material average grain size was measured and analyzed along the rolling direction by the ImageJ software program and the linear intercept method, which was found to be 33.95 ± 9 µm, as shown in Fig. 5b.In addition, the volume fraction of the η-phase (MgZn 2 ) was measured using the ImageJ software program, and was found to be 3.86%, as shown in Fig. 5c.Therefore, the solidification cracking susceptibility of AA7075 basic material can be reduced to acceptable levels by modifying it with excess silicon (using the 4xxx series Al-Si filler alloys) or excess magnesium (using the 5xxx series Al-Mg filler alloys) 35,36 .To inhibit the propagation of hot cracks during and after the welding process, a 4XXX filler metal wire containing Si is typically utilized.In other situations, the strength of the welded sample is enhanced by using the 5XXX filler metal 37 .Figures 6, 7, 8 display the optical microscopy images for various welding regions of the AA7075 Al alloy welded by TIG welding with different welding wires.Additionally, the BM long, flat micrograins are oriented in the direction of the rolling mill as a result of deformation, and when welding occurs, the impact of fusion and heat input causes the grains to become coaxial and larger in size.This phenomenon is clearly observed in the transition regions of the welding lines, as shown in Figs.6a, 7a, and 8a 38 .However, in the HAZ, as shown in Figs.6a, 7a, and 8a, the microstructure is mainly influenced by two types of solid-state reactions 39 : (a) the dissolution of precipitates and the growth of grains in areas that experience higher peak temperatures and (b) the partial dissolution of precipitates and the transformation from a metastable phase to a stable phase in regions that experience lower peak temperatures.The AA7075 Al alloy has a wide freezing range, resulting in the presence of coarse columnar grains and partial melting at the grain boundaries in the region between the WZ and HAZ.
A fine grain structure of the welded joint filled with ER5356 may be seen in the WZ, as seen in Fig. 6b,c, with a grain size of 17.91 ± 4 µm, an area volume fraction of MgZn 2 of 5.83%, and an average grain size of the MgZn 2 phase of 3.43 µm 40 , as seen in Fig. 6d.Fast cooling rates in the related weld are attributed to the generally fine weld zone microstructure and chemical composition of ER5356 filler metals.In addition, the WZ, where melting and resolidification occur after welding, as well as a zone damaged by the heat of the welding operation, must be considered.The main factors influencing the creation of the weld metal in the weld zone are diffusion, temperature gradient, and solidification growth rate.The formation of a eutectic constituent at the final stage of solidification, which removes the majority of the materials required for precipitation reactions from the matrix, limits precipitation aging 41 .On the other hand, the grain size of the welded joint filled with ER4043 filler metal is 22.33 ± 7 µm (Fig. 7b,c) with an area volume fraction of MgZn 2 of 4.77%, and the grain size of MgZn 2 is 4.77 µm, as seen in   7d, while the grain size for the welded joint filled with ER4047 filler metal is 42.31 ± 11 µm (Fig. 8b,c) with an area volume fraction of MgZn 2 of 8.76% and grain size of MgZn 2 phase of 7 µm, as seen in Fig. 8d.

Mechanical properties of weldments
Figure 9 compares the experimental results for the mechanical behavior of the BM and welded specimens as a function of different welding wires (ER5356, ER4043, and ER4047).The welded specimens with welding wires of ER5356 have superior ultimate tensile stress (UTS), yield stress (YS), and percentage of elongation (El%) of 387.8 ± 11 MPa, 305 ± 7.4 MPa, and 6.5 ± 0.2% with an efficiency of 72% (Fig. 10) as compared to the one fabricated with rod ER4043 having UTS, YS, and El% of 337.53 ± 6.5 MPa, 278 ± 4.6 MPa, and 8.3 ± 0.35% with an efficiency of 62.7% (Fig. 10) and ER4047 having UTS, YS, and El% of 297.94 ± 8.3 MPa, 249 ± 6.2 MPa, and 5.65 ± 0.3% with an efficiency of 55.3% (Fig. 10), respectively.This is due to the difference in the composition and solidification microstructure of the filler metals.Additionally, the weld zone grain size of the sample filled with ER5356 filler metal was smaller than that of the samples filled with ER4043 and ER4047 filler metals, as indicated by 27,42 .The addition of rod ER5356 in the welded sample results in a more condensed pressure, which in turn enhances the bond strength in contrast to using filler metals ER4043 and ER4047.On the other hand, the use of fillers ER4047 and ER4043 in the samples caused a significant increase in grain size, leading to a decrease in the mechanical properties 42 .According to the Hall-Petch equation, tensile stress is inversely related to grain size 43 .The ductility of the welded samples using ER4043 filler metal was roughly 27.7% and 46.9% higher than that of the welded joints using ER5356 and ER4047 fillers, respectively.The reformed eutectic phase plays a crucial role in controlling the ductility.Fracturing started in the plastic region of the stress-strain curves for all the samples and finally occurred in the WZ.Meanwhile, the UTS-to-YS ratio can be used to calculate a material's strainhardening capacity (Hc) 44,45 .Hc was modified to a normalized parameter by Afrin et al. 45 The material's ability to resist deformation under tension, known as the tensile hardening capacity, is somewhat dependent on its strength, specifically its Hc value, which is directly correlated with the material's strength 46 .The hardening capacity of metals is principally determined by the interaction between dislocations rather than relying on the grain size for grain boundary strengthening 47 .Hence, Fig. 11 displays the Hc of the joints that were welded using various welding wires.It was discovered that the base metal had the highest Hc value of 0.32, whereas welded joints utilizing ER4047 fillers had the lowest Hc value of 0.2. Figure 12 displays the fractography (1)   of the weld specimens obtained by scanning electron microscopy at high magnifications.Dimples, tear ridges, ductile dimples, and cleavage facets formed the fractured surface caused by the base metal and welded joints of the ER4047 welding wire, as shown in Fig. 12a,c.Because ER4047's shattered surface of the base metal and welded joints are dominated by transgranular ductile dimples.In contrast to ER5356, which has a fragmented surface with fine dimples, a dominant cup, and a cone structure, as shown in Fig. 12b.A small percentage of ripping ridges indicates that the weld may have torn relatively early in its development, when it may have been one of the primary causes of failure.However, ductile ripping dominated the failure mechanism as the tensile load increased.

Influence of different welding wires on the macrohardness of welded samples
Figure 13 illustrates the macrohardness variations in the different welding zones from the BM, HAZ, and WZ for the AA 7075 welded samples using various welding wires from ER5356, ER4047, and ER4043.All the welding wires exhibited a similar trend in the macrohardness profile for all the produced welds.In addition, different transition zones of the weldment show a stark variation in hardness values in the WZ, HAZ, and BM, as shown in Fig. 13.The variations in macrohardness values are caused by differences in the chemical composition, morphology, and microstructure distribution of various welding wires.Additionally, the use of welding wires with different chemical compositions is mostly to blame for the noticeable decrease in WZ hardness when compared to the BM and HAZ.9][50][51] .It is clear that the hardness values of the WZ for all welds produced using different welding wires of ER5356, ER4043, and ER4047 are relatively changed compared to the BM and HAZ.Consequentially, the hardness values in the WZ filled by ER5356 are higher than those in the WZ welded by ER4043 and ER4047, which is primarily because the former's microstructure is dominated by a coarse grain structure.The difference in the hardness values can be attributed to the distinct groups of elements in the filler rods.The 5xxx series (ER5356) from the Al-Mg group primarily contains magnesium as the major alloying element, whereas the 4xxx series (ER4043 and ER4047) from the Al-Si group predominantly contains silicon, which has a lower hardness.The test results indicate that the 7xxx series from the Al-Mg-Zn group exhibits higher hardness values.It was observed that the hardness values in the AA7075 BM and HAZ were greater than those in the WZ for all the welded samples.This reduction in hardness can be attributed to the decreased number of dislocations and exposure to high temperatures in this region, which reduces the hardening effect of the strengthening solutions, as supported by the literature 50,[52][53][54] .
The formation of an unmixed zone and the migration of alloying elements from the weld and AA 7075 base material contribute to a sudden increase in hardness in the HAZ.The AA 7075 side of the HAZ had a chemical composition similar to that of the base and weld material.The slight variation in hardness in this region is mainly due to grain coarsening resulting from the different temperatures in the HAZ.The high hardness of the HAZ on the AA 7075 side makes it more susceptible to failure.Compared to the ER4043 and ER4047 filler welds, the unmixed zone of the ER5356 filler was relatively small, indicating its superiority in this regard.The limited width of the high-hardness transition zone in the ER5356 filler did not negatively impact the overall performance of the weld.Consequently, the overall structural integrity of the weld remained intact.

Influence of different welding wires on the toughness of welded samples
Figure 14 illustrates the influence of various filler metals on the impact toughness of the welded samples.As a result, the impact toughness values of all welds were higher than 100 J in every welded sample, which is a requirement for the structural integrity of welds of a comparable kind.Compared to fillers ER5356 and ER4043, filler ER4047 offers greater impact toughness.The coarse grain structure and skeleton microstructure of the eutectic in ER4047 are known to reduce the likelihood of solidification and liquefication cracking and to give the weld a significant amount of impact toughness.The weld with the ER4047 rod exhibited an approximately 33.7% increase in impact toughness in the WZ compared with the BM.Joints welded using ER5356 welding wires exhibit lower impact toughness compared to other filler metals and are hence more vulnerable to failure.The HAZ structure and weld have nearly identical elemental compositions.Furthermore, the microstructural phases exhibit some resemblance to the alterations that arise as a result of tempering, which is attributed to the thermal fluctuations experienced during welding.

Limitations and future directions
While this research offers valuable information about the consequences of using different filler metals on the AA7075 TIG welding process, there are several limitations that must be noted.These findings are relevant only to the AA7075 alloy and the three filler metals tested (ER5356, ER4043, and ER4047) under the specific welding conditions investigated.It is important to consider that variations in welding parameters, such as heat input or travel speed, could affect the reproducibility of the results.Furthermore, the study did not investigate the stability of the observed properties under long-term aging.Also, the research was focused on particular mechanical

Figure 5 .
Figure 5. Microstructure images for initial material AA7075 Al alloy.

Fig.
Fig.7d, while the grain size for the welded joint filled with ER4047 filler metal is 42.31 ± 11 µm (Fig.8b,c) with an area volume fraction of MgZn 2 of 8.76% and grain size of MgZn 2 phase of 7 µm, as seen in Fig.8d.

Figure 9 .
Figure 9. Mechanical behavior for welded joints by various welding wires.

Figure 14 .
Figure 14.Impact toughness for welded joints by various welding wires.

Table 1 .
The composition for Al-Zn-Mg alloy and welding wires, wt%.

Table 2 .
The mechanical properties for Al-Zn-Mg alloy and welding wires.

Table 3 .
2 CuMg), T-phase (Al 2 Mg 3 Zn 3 ), and θ-phase (Al 2 Cu).Insoluble primary Fe-rich phases (Al 7 Cu 2 Fe, Al 3 Fe, and α-AlFeSi), Si-rich phases (Mg 2 Si The visual examination report of TIG butt joints produced of aluminum alloy AA7075 that were welded using various welding wires.The welding path width is 8.42 mm Face reinforcement is about 1.28 mm ER4047 200 × 200 × 6 The welding path width is 9.81 mm Face reinforcement is about 1.56 mm